This invention relates generally to the use of a fluidized bed combustor for heating air and more specifically to the use of an atmospheric circulating fluidized bed combustor in a Compressed Air Energy Storage (CAES) system, thereby reducing or eliminating the need for conventional fuel combustors as well as premium fuels associated with such combustors.
Compressed Air Energy Storage power plants have become effective contributors to a utility's generation mix as a source of peaking or intermediate energy and spinning reserve. CAES plants store off-peak energy from relatively inexpensive energy sources such as coal and nuclear baseload plants by compressing air into storage devices such as underground caverns or reservoirs. Such underground storage can be developed in hard rock, bedded salt, salt dome or aquifer media. Following off-peak storage, the air is withdrawn from storage, heated, combined with fuel and expanded through expanders, i.e., turbines, to provide needed peaking/intermediate power. Since inexpensive off-peak energy is used to compress the air, the need for premium fuels, such as natural gas and imported oil, is reduced by as much as about two thirds compared with conventional gas turbines.
Compressors and turbines in CAES plants are each connected to a generator/motor device through respective clutches, permitting operation either solely of the compressors or solely of the turbines during appropriate selected time periods. During off-peak periods (i.e., nights and weekends), the compressor train is driven through its clutch by the generator/motor. In this scheme, the generator/motor functions as a motor, drawing power from a power grid. The compressed air is then cooled and delivered to underground storage.
During peak/intermediate periods, with the turbine clutch engaged, air is withdrawn from storage and provided to a combustor. The combustor combines the pre-heated compressed air with a fuel, such as No. 2 fuel oil, and expands the mixture of fuel and compressed air in a turbine, which provides power by driving the generator/motor. In this scheme, the generator/motor functions as a generator, providing power to a power grid. To improve the CAES heat rate, waste heat from a low pressure turbine exhaust is used to pre-heat high pressure turbine inlet air in a recuperator.
For a more complete discussion of CAES systems, see Nakhamkin, M. et al. "Compressed Air Energy Storage: Plant Integration, Turbomachinery Development", ASME International Gas Turbine Symposium and Exhibition, Beijing, Peoples' Republic of China, 1985 and Nakhamkin, M. et al. "Compressed Air Energy Storage (CAES): Overview, Performance and Cost Data for 25 MW to 220 MW Plants", Joint Power Generation Conference, Toronto, Canada 1984, both incorporated herein by reference.
Although CAES systems reduce the need for premium fuels by as much as about two thirds compared with conventional gas turbines, premium fuel such as natural gas or fuel oil is still required by combustors in conventional CAES systems. However, the use of premium fuels is severely restricted by institutional issues and requires special exemption from the Powerplant and Industrial Fuel Use Act of 1978.
It is therefore desirable to provide a peaking/intermediate system which does not require any premium fuel and which operates on the combustion of low grade fuels such as coal, municipal solid wastes, peat and the like.
Combustors which operate on low grade fuels are known. Illustrative of such a combustor is a system comprising a combustion chamber having a combustion air inlet, a fluidizing air inlet, a fuel/solid particle inlet and a heat transfer vessel such as a tubular coil containing a medium to be heated. In such a fluidized bed combustor, a quantity of solid particles are kept in turbulent motion by a fluidizing forced air stream input via a fluidizing air inlet. Such solid particles, illustratively limestone, in turbulent motion are known as a fluidized bed. Fuel may be added via the fuel inlet to such bed and combined with air from the combustion air inlet to provide heat to the heat transfer vessel submerged within or in close proximity to such bed. Hot solid particles and gases churn and surround the heat transfer vessel resulting in high heat transfer coefficients. Hot gases downstream from the fluidized bed similarly may be used to provide heat to an additional heat transfer device.
Environmentally, fluidized bed combustion is desirable since it permits the burning of coal in a bed of hot limestone. This is beneficial due to the availability of coal in the United States as well as the easily disposed dry calcium-sulfate waste product produced by the absorption of sulfur dioxide (released by burning coal) by calcium oxide from the calcination of limestone. Furthermore, emission of nitrogen oxides (NO.sub.x) is minimal due to the relatively low combustion temperatures generally associated with FBCs (for example, 1400.degree. F.-1700.degree. F.).
Despite the above-enumerated advantages of Fluidized Bed Combustors (FBCs), difficulty has been encountered in their application to power generations systems. Illustrative of such FBC applications is U.S. Pat. No. 3,913,315 in which a FBC is utilized to heat compressed air of approximately 50 atm. However, this system requires the use of a pressurized fluidized bed combustion chamber in which the operating pressure of the combustion chamber is within one atmosphere of the compressed air pressure within the heat transfer device. Such Pressurized Fluidized Bed Combustors (PFBCs) have presented technological difficulties associated with elevated temperature operation. Additional U.S. Pat. Nos. disclosing known fluidized bed combustion devices are 4,476,674; 4,380,147; 4,223,529 and 4,116,005.